CiviCRM

Excellent choice for small-medium organisations because of zero setup and ongoing fees.

Traditional Direct Debit bureaus can't compete until you get over 500 monthly direct debits with an average donation beyond £50/month.

Great for developers: built on modern REST API; webhooks that are fully logged; fully functioning sandbox accounts for testing.

But...

Donors will be aware of the GoCardless brand both in the sign-up process and on their bank statements, although that can also include your organisation's name too. You can pay to use your own BACS SUN number but they charge a lot for this.

Quirk: it can use Direct Debits for single payments.

Room for improvement: it does not include details of fees charged; manging multiple accounts requires a different email for each account; they had no comment in terms of any ethical or sustainability policies.